What is the meaning of the word circular reasoning?

Circular reasoning (Latin: circulus in probando, "circle in proving"; also known as 'circular logic') is a logical fallacy in which one begins arguing in the wrong end of a premise. Because all propositions are proved based ultimately on the original assumption including the original assumption no valid conclusion can be reached.

Circular reasoning is often of the form: "A is true because B is true; B is true because A is true." Circularity can be difficult to detect if it involves a longer chain of propositions.